The long-term goal of this renewal application is to understand the mechanisms responsible for generating and maintaining antiviral CD8 T cell responses required to contain persistent smoldering viral infections. Persistent virus infections may be categorized into those that are chronically viremic (e.g., HIV-1) and those that maintain themselves by repetitive cycles of reactivation of lytic replication from a noninfectious latent state (e.g., herpesviruses). Polyomaviruses (PyV), silent residents of the virome of many vertebrates including humans, persist in a low-level infectious state. However, under conditions of immunosuppression human PyVs can incur life-threatening disease. BK virus is a significant cause of kidney transplant dysfunction and loss. A major cause of mortality in the HIV/AIDS population, JC virus-induced Progressive Multifocal Leukoencephalopathy (PML) has recently emerged in individuals receiving humoral immunotherapies for autoimmune and inflammatory diseases (e.g., multiple sclerosis, Crohn's disease, and severe psoriasis). Several new human PyVs have also recently been isolated from nonbacterial respiratory infections and as a likely causative agent for an aggressive fatal cutaneous malignancy. No effective therapies for PyV infection are available. Due to the tight species specificity of Polyomaviridae that restricts infection to natural animal reservoirs, we have an incomplete understanding of the immune mechanisms needed to keep these smoldering infections in check. Using mouse polyomavirus (MPyV), we discovered that na?ve virus-specific CD8 T cells are recruited during persistent infection. Data generated in the last funding cycle reveal that temporal differences in recruitment dramatically modulate the quality of the memory T cell compartment. We hypothesize that dynamic changes over the course of infection govern whether MPyV-specific CD8 T cells differentiate into competent memory cells, which, in turn, will be reflected at their levels of gen expression and the TCR-CD8-peptide:MHC tri-molecular interaction. Understanding the mechanisms that control the generation of durable anti-MPyV CD8 T cell memory may help in the development of interventions to enhance immunity to human PyV infections. We will test the above hypothesis in the following three Specific Aims: Aim 1: To define determinants regulating the contribution of acute vs. persistent infection-recruited antiviral CD8 T cells to the memory population. Aim 2: To determine molecular signaling mechanisms underlying the functional differences between acute and persistent infection-recruited MPyV-specific CD8 T cells. Aim 3: To define molecular differences between MPyV-specific memory T cells recruited during acute and persistent infection.